This invention relates to a method for modifying the toxicity of DNA reactive cross-linking agents such as nitrogen mustard. More particularly, the invention relates to treating cells exposed to DNA reactive cross-linking agents with one or more endocytotically transported electrophyllic or chemically reactive materials such as serum albumin to decrease the toxicity of the drug to the exposed cells.
Advances in the treatment of tumors have come quickly in the last several decades. One common method of treating cancer patients is through "chemotherapy", therapy in which the cancer patient is treated with certain drugs capable of killing cancer cells. A class of drugs useful for chemotherapy are nucleophillic materials capable of being absorbed into the nucleus of a cell endocytotically and there cross-linking the DNA, referred to hereinafter as DNA cross-linking agents. These cross-linking agents are bifunctional compounds, possessing at least two nucleophillic centers capable of covalently bonding to electrophyllic centers in the DNA macromolecule. Cross-linking the DNA in the cell serves to kill the cell as it no longer has normal nuclear functions.
Nitrogen mustards (such as nitrogen mustard itself, methyl-N-bis-(2-chloroethyl)-N-ethylamine, commonly referred to as HN2 or mustargen) are DNA cross-linking agents and were the first chemotherapeutic agents applied to the treatment of tumors. It has been reported that HN2 taken up by the cell travels through the cytoplasm to the nucleus and bifunctionally alkylates DNA. Rutman, R. J., Steele, W. J., Price, C. C., "Experimental Chemnotherapy Studies II. The Reactions of Chloroquine mustard (CQM) and Nitrogen Mustard (HN2) With Ehrlich Cells," Cancer Res., 21:1134-1140, 1961. The cytostatic effect of HN2 is known to be proportional to the degree to which DNA in the cell is crosslinked. Chun and Rutman, et al., Cancer Research, (1966); Lyons, R. M. and Goldenberg, G. J., "Active Transport of Nitrogen Mustard and Choline by Normal and Leukemic Lymphoid Cells," Cancer Res. 32, 1679-1685, 1972. It has been reported that the alkylation and cross-linking of DNA by HN2 proceeds as two pseudo-first-order reactions; at 37.degree. no unstable displaceable intermediates are observed, but the cross-linking reaction can be interrupted by nucleophillic competition. Rutman, R. J., Chun, E. H. L. and Jones, J., "Observations on the Mechanism of the Alkylation Reaction Between Nitrogen Mustard and DNA," Biochim. Biophys. Acta. 174:663-673 (1969).
A well known problem inherent in the use of chemotherapeutic agents such as nitrogen mustard, however, is the fact that the chemotherapeutic agents are relatively nonselective. They are toxic not only to the target cancer cells but also to normal, non-cancerous cells such as those in bone marrow and other rapidly dividing tissues and organs, a fact which can severely limit the clinical use of the agents. Sensitive patients receiving such drugs may require a substantial reduction in dose, to less effective levels, because of toxic side effects such as extreme depression of white blood cell count. With repeated courses of treatment, there is a threat of cumulative injury to the nontarget cells. There is therefore a clear need for a method which would allow one to utilize effective chemotherapeutic agents in the treatment of tumors without causing serious side effects to the patient.
It is an object of this invention to provide a method for modifying the cytotoxic effects of DNA reactive cross-linking agents such as nitrogen mustard. It is a further object of this invention to provide a method by which DNA reactive cross-linking chemotherapeutic agents can be used effectively to treat tumors while significantly reducing the toxic side effects to non-target cells. These and other objects will be clear from the following description of this invention.